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Not AvailableIn the estuarine water bodies, diurnal and tidal cycle studies on physico-chemical and biological parameters give better understanding of the fluctuation in the quality of incoming and outgoing waters with reference to tidal effects. In such studies of true estuaries, high and low tide data are normally differentiated based on the direction of water flow (period of rise and fall for high and low tide phases respectively) which do not always give satisfactory results of their effects on hydrographic and hydrobiological parameters. In the present study, an attempt was made to compare the high and low tide data separated based on the local tidal amplitude with that of the direction of water flow using salinity as the tide dependent parameter. When the local tidal amplitude was taken into account, bihourly data collected during the period with more than 50% of local tidal amplitude were treated for high tidal effect and the data falling below 50% of it were treated for low tidal effect. The data on salinity and primary productivity obtained from Chandragiri and Murad estuaries of north Kerala were examined to study the high and low tidal effects. The results confirmed that the proposed methodology based on 'tide water level' (local tidal amplitude) would give reliable results than the existing method (based on the direction of water flow) in the diurnal studies of estuarine ecosystems. Separation of low and high tide data on primary productivity based on the proposed methodology revealed that low tide water was relatively more productive than high tide waer especially during postmonsoon period in these two estuaries along the southwest coast of India.Not Availabl